1. Field of the Invention
The present invention relates to a process for producing a polyurethane foam by the reaction of a polyol with a polyisocyanate in the presence of an amine catalyst, a blowing agent, and a foam stabilizer. Particularly, the present invention relates to a process for producing a flexible or rigid polyurethane foam by use of an amine catalyst having a long-chain aliphatic group in the molecule having less odor and lower volatility and exhibiting high curing rate and improved moldability with a formulation comprising reduced chloro fluorocarbons and increased water as the blowing agent. The flexible polyurethane foam herein includes semi-rigid polyurethane foams, since the same polyol is used.
2. Description of the Related Art
Polyurethane foams are widely used as flexible foams for automobile seat cushions, mattresses, furniture, etc.; as semi-rigid foams for automobile instrument panels, head rests, arm rests, etc.; and as rigid foams for refrigerators, building materials, etc.
In recent years, polyurethane foams are desired to be improved in the curing rate, the demolding time, and the foam yield in order to reduce polyurethane production cost and foam productivity. The polyurethane foam is formed mainly through two reactions including a reaction of a polyol with an isocyanate (gelling reaction) to form a urethane linkage, and a reaction of an isocyanate with water (blowing reaction) to form a urea linkage and to evolve carbon dioxide. The catalyst affects not only the reaction rates but also the curing rate, the moldability, the flowability of the foam, foam density and the properties of the foam.
In recent years, reduction of use of chlorofluorocarbons, so-called CFCs such as trichloromonofluoromethane, and dichlorodifluoromethane which causes ozone layer destruction, improvement of working environment, and reduction of emission of evaporative substances from products, and the like environmental problems are attracting attention. Formulations have been investigated in which CFC as the blowing agent is reduced and water is increased. The increase of water as the blowing agent, however, lowers greatly the curing rate of the foam, and impairs significantly the moldability as well as the adhesive strength and dimensional stability of the rigid foam. Accordingly, a catalyst is strongly requested which is suitable for the formulation containing a decreased amount of CFC and increased amount of water.
As the catalyst for polyurethane production, organometallic compounds and tertiary amines are known. The tertiary amine is widely known to be an excellent catalyst for production of polyurethane. The tertiary amine employed industrially for polyurethane production is exemplified by triethylenediamine, N,N,N',N",N"-pentamethyldiethylene-triamine, N,N,N',N'-tetramethylhexamethylenediamine, N-methyl-N'-(2-dimethylaminoethyl)piperazine, triethylamine, N-methylmorpholine, N-ethylmorpholine, dimethylcyclohexylamine, dimethylbenzylamine, etc. Of these polyurethane production catalysts, triethylamine, N-methylmorpholine, N-ethylmorpholine, and N,N,N',N'-tetramethylhexamethylenediamine, which have a relatively low molecular weight and exhibit high volatility, are widely used owing to satisfactory effects on curing rate, moldability, and foam properties.
The polyurethane production catalysts having been developed up to now have various disadvantages as follows. Tertiary amines emit offensive strong odor. In particular, amines of relatively low molecular weight such as N-methylmorpholine, N-ethylmorpholine, triethylamine, N,N,N',N'-tetramethylhexamethylenediamine, dimethylcyclohexylamine, etc. emit extremely pungent odor, and worsen the working environment greatly by the strong odor in the foaming process. Particularly when a larger amount of an amine catalyst is used, blurring of eye (eye rainbow) or a like symptom is frequently caused. Furthermore, the resulting polyurethane product has also residual offensive odor, or emits the amine to impair the value of the product, disadvantageously.
Conventionally, CFCs have been used as the blowing agent in production of flexible or rigid polyurethane foams. In recent years, destruction of the ozone layer by CFCs has come to be noticed. Consequently, reduction of use of CFCs is an urgent problem. For this purpose, polyurethane foam formulations have been studied which use a less amount of CFC and an increased amount of water as a substituting blowing agent. The increase of water in place of the CFC, however, give rise to various technical problems. In production of flexible polyurethane foams, water reacts rapidly with the isocyanate with violent gas evolution, which causes difficulties in foam formation, or deterioration of moldability such as considerable drop of curing rate at the surface and the interior, exfoliation of the skin layer, void formation, roughening of cells, and so forth. On the other hand, the production of rigid polyurethane foams involves serious problems such as the increase of crystalline urea linkage which is formed by the reaction of water with isocyanate causes brittleness of the foam surface, resulting in drop of bonding strength, increase of friability, as well as the drop of dimension stability, and other disadvantages.
In order to solve such problems, compounds having an active hydrogen-containing group capable of reacting with an isocyanate, and amine catalysts having a relatively high molecular weight are disclosed. The compounds having an active hydrogen group include amine compounds such as N,N-dimethylethanolamine, N,N-dimethylaminoethoxyethanol, etc. However, N,N-dimethylethanolamine and N,N-dimethylamino-ethoxyethanol have also disadvantages of offensive odor. Moreover the rapid deactivation during the foam-formation process by the reaction with isocyanate causes insufficient catalytic activity at the later period of polyurethane formation reaction, low curing rate, longer demolding time in a mold tool, and unsatisfactory moldability, and so forth.
Amine catalysts having a relatively high molecular weight comprising an aliphatic monoamine of 6 to 10 carbons are disclosed to be useful in formulations as amine catalyst for rigid foams which employ a CFC as a blowing agent with a small amount of water. Aliphatic monoamines of 11 carbons or higher are shown to have disadvantages of significantly low catalytic activity, inferior properties of foams, and significantly low surface curing properties (JP-A-58-93715).
The present invention has been accomplished to solve the above problems.